Thermoelectric properties and figure of merit of Cu2ZnSnS4

Abstract

Thermoelectric (TE) power is a leading technology for addressing energy waste, and the quaternary sulphide Cu₂ZnSnS₄ (CZTS) is of considerable interest for lightweight and flexible thin-film devices. However, the large variation in the figure of merit, ZT, obtained in experimental studies highlights the need for a more comprehensive characterisation of the material properties. In this work, we apply a fully ab initio modelling approach to obtain high-quality reference predictions of the thermoelectric properties and ZT of CZTS. We find that CZTS has large bulk κ_latt, due to long phonon lifetimes, which can be minimised by nanostructuring. We predict significantly higher electrical conductivity, Seebeck coefficients and power factors with p-type (hole) doping compared to n-type (electron) doping. The electrical properties are less sensitive to crystallite size than the κ_latt, and we predict an industry-standard high-temperature ZT > 1 should be achievable for nanostructured p-type CZTS with crystallite sizes below 10 nm. Comparing our predictions to measurements also suggests that modelling heavily-doped CZTS or CZTS-based solid solutions may yield valuable insight into how the ZT can be optimised further.